New tickets as of 2008.
It costs CAD$7 to buy the OPUS Card. It is a micro-chip based card, and it is necessary if you wish to buy weekly or monthly passes. OPUS can store multiple single tickets on it also.
The à la carte tickets are paper-based with a magnetic strip. These have no additional cost other than amount of fare.
There are some minor disadvantages with the OPUS card for tourists:
1) Unlike other cities (Boston for example), Montreal is charging a fee to buy the OPUS card.
2) The OPUS card has an expiry date - which is four years. This means that every four years a new card has to be obtained. If your old card has not expired for less than 6 months, you can get a replacement for free at the Berri-UQAM office.
3) Unlike many other cities' cards, the OPUS card does not store cash, so it cannot be used for multiple passengers.
For tourists visiting for multiple weeks, there is no choice but to get a OPUS card since the weekly pass (Mon-Sun validity) will not be available in paper-ticket form. Monthly pass is also only available on OPUS.
For tourists who need only 1 or 3 days passes, or only evening/weekend tickets, there is no need to get OPUS card and you can just buy paper tickets.
One problem for US tourists is that credit cards will not work in the automated machines. While the STM web site claims credit cards are allowed, a bunch of cards I tried were all denied. It was later confirmed in a newspaper report that STM has disabled use of non-Canadian credit cards.
On the bus, only coins are accepted, and only single tickets can be bought.
One very good about about STM fares is that they have many weeknight and weekend options, which are all incredible deals for tourists. These don't require a OPUS card, and can be purchased as paper tickets at the same price.
24-hour travel, 6PM+ travel for that night, and a Fri 6PM to Mon morning weekend ticket are great options. And there is a 3-day consecutive ticket expires midnight on the third day (so it is not a 72-hour pass).
One tricky thing about buying the weeknight or weekend passes on the OPUS card is that it confuses the system when that ticket expires - for example, after buying a weekend pass, if on Mon at 6PM you try to buy the 6PM+ night ticket on the OPUS card, it will not allow that. Something about the system not realizing that the old pass expired Mon morning. So OPUS card cannot be used to buy Mon evening pass. Easy fix: if this happens, just buy the paper ticket pass instead. Luckily, unlike the extremely tourist-unfriendly fares in Boston, Montreal STM prices are same for both paper tickets and OPUS card tickets.
When coming or going to the airport, best to buy the 1-day or 3-day or longer pass. It works on the 747 Express Airport bus. Be careful of buying a ticket for this - there are two versions of the ticket, both cost the case. But one version only allows a single trip on this bus, the other allows travel on metro, bus, for a 24 hour period (i.e., the second one is same as the 1-day pass. This is very confusing, and there are stories of tourists getting the wrong ticket on the bus, so better to ask for a 1-day pass rather than a 747 Bus ticket.
Main web sites:
OPUS Card
À la carte Tickets